Earthquake or earthquake are sudden, passing tremors that occur on the Earth's surface, caused by underground shocks of crustal rock plates 300m below the ground.

Other reasons considered are gas displacements (mainly methane) and volcanic activities. There are two types of earthquakes: those of natural origin and the induced ones.

Most earthquakes are of natural origin from Earth, called tectonic earthquakes. The strength of the tectonic plates slips and can move apart, collide or slide together. With these forces the rocks change to their point of elasticity, after which the rocks begin to break and release an accumulated energy during the process of elasticity. Energy is released through seismic waves across the earth's surface and interior.

In most cases, the vibrations are very weak, being perceived only with the aid of special apparatus. But some earthquakes can cause harmful effects to man such as injury, death, financial and social damage, collapse of buildings etc.

Consider this comparison: if you bend the plastic ruler slightly with your hands, you will feel a force (a strain) of that goal against your hands. If you release one end, the tension will make the ruler vibrate.

Most earthquakes occur when certain tension at the boundary between two tectonic plates is released. Two moving plates can touch each other, exert pressure against each other, and become trapped with each other. At any given moment, the accumulated force between them can overcome friction, causing a rapid slip: one plate slips along the other, which releases the accumulated energy. This energy triggers "shock waves," called seismic waves, that spill over rocks and cause earthquakes.

There are also induced earthquakes that are compatible with human-made anthropic action. They originate from explosions, mining of minerals, water or fossils, or even from falling buildings; but have much lower magnitudes than tectonic earthquakes.

The consequences of an earthquake are:

  • Ground vibration,
  • Fault opening,
  • Landslide,
  • Tsunamis,
  • Changes in the earth's rotation.

It was from the 1900s that the main measurement scales we came to know were born. The best known is the Richter scale, developed by the American Charles Richter (1900-1985). The Richter scale ranges from 0 to 9.5 or more points. The last level may vary: it will depend on the strength of the largest earthquake to date.

The most intense shake-up ever recorded in the twentieth century reached 9.5 points on the Richter scale and occurred in Chile in 1860. Turkey's in 1999 reached about 7.4 points on the Richter scale. The earthquake that caused the highest number of deaths occurred in China in 1556 - 830,000 dead.

The most earthquake-prone regions are regions near tectonic plates such as western South America where the Nazca plate and the South American plate are located; and in regions where new plates are formed, such as in the Pacific Ocean where the Fire Belt is located. The length of an earthquake fault can range from centimeters to millions of kilometers, such as the San Andreas fault in California, United States.

In the United States alone there are about 13,000 earthquakes a year ranging from approximately 18 major earthquakes to one giant earthquake, the rest being mild or even unnoticed.